1. Field of the Invention
This invention relates to the field of air pollution control and more particularly to the removal of acid gases from gas streams resulting from the operation of various types of processes including processes for the production of heat or power, and industrial, chemical, combustion, material handling and other processes.
Although Congress passed the Clean Air Act of 1970 and, since that time, has passed additional legislation directed toward the improvement of air quality, the quantity of acid gases, for example, sulfur dioxide (SO.sub.2), entering the air has not decreased as much as expected. Currently about 20 million tons of sulfur dioxide are released per year into the atmosphere from sources within the United States. While the sulfur dioxide emitted from power plants and ore smelters is a major source of acid rain, nitrous oxides, which produce nitric acid in the atmosphere, are generated by internal combustion engines and are also a substantial contributing factor to the problem. Incinerators, for example, may also emit chlorine-containing gases which form hydrochloric acid in the atmosphere as well as sulfur dioxide.
During the past 25 years a number of solutions to the acid rain problem have been suggested but these have foundered on the twin shoals of money and politics. Although wet scrubbers have been installed on a number of power plant boilers, incinerators and smelters, these scrubbers are relatively expensive to operate and maintain and do not generally improve the operating efficiency of the plant. They thus add cost to the process which, for some operations, may be prohibitive. Politically it has proven difficult to impose such higher costs on the energy industry or other basic industries in a given area, such as the midwest, where the result might be to diminish the industrial base. Similarly, it has been politically difficult to discourage the use of high-sulfur eastern coal in eastern or midwestern power plants in favor of the lower-sulfur western coals.
While certain types of wet scrubbers are capable of removing a relatively high percentage of the acid gas contained in a polluted gas stream, the result may be to trade air pollution for water pollution since the captured acid gases are entrapped by the scrubbing water. Further treatment of the scrubbing water is needed before it can be reused or discarded. Moreover, many industrial processes, including ore smelters, may be located in areas where water is scarce.
Attempts have been made to use dry or damp processes to capture acid gases. Such processes ordinarily employ powdered lime or limestone but usually are incapable of removing more than about 60% to 80% of the SO.sub.2 contained in a gas stream.
It thus appears that there is a need for a process and apparatus capable of reliably removing substantially all of the acid gas from a gas stream in the dry condition and at a relatively low cost. The present invention relates to such a process and apparatus.
2. Description of the Prior Art
Many polluted gaseous streams contain both particulate matter which may vary widely in size, shape and chemical composition and noxious gases such as sulfur dioxide, hydrogen sulfide, chlorine, or various nitrous oxides. The noxious gases are usually acidic in nature and may be reacted with basic chemicals to form sulfates, carbonates, chlorides or nitrates which, if dry, can be disposed of without great expense or difficulty. Some pollution control apparatus is effective to capture both particulate and acidic gases while other apparatus is effective on only one of these pollutants.
One well-known category of equipment is the fabric filter which is usually in the form of a plurality of bags mounted in parallel to form a baghouse. The baghouse is capable of trapping particulate as small as about 1.0 micron in size but it is not well adapted to the removal of gaseous pollutants. Efforts have been made to include a powdered basic material such as lime in the bags but this has not proved to be entirely satisfactory.
Another category of equipment is the mechanical separator. This usually takes the form of a centrifugal separator or cyclone. Sometimes a number of small cyclones are connected in parallel since the efficiency of a cyclone is inversely proportional to its diameter while its capacity is directly proportional to its diameter. As the cyclone depends upon centrifugal forces to separate the particulate from the gas stream, it is relatively ineffective for the separation of gaseous materials. The cyclone is most effective for particulate in excess of about 10.0 microns in size.
A third category of gas stream cleaners is the electrostatic precipitator in which the gas stream passes between oppositely charged plates after being charged to one polarity. The particulate will then be attracted to the plate charged to the opposite polarity. While the electrostatic precipitator is widely used, it is most effective for particulate above about 2 to 3 microns in size. Additionally, the precipitator is relatively expensive to operate and maintain; its performance tends to deteriorate with time; and it is sensitive to combustible material in the gas streams. Finally, the precipitator is not effective with respect to acid gases, such as sulfur dioxide.
The above equipment is generally of the dry type. A variety of wet scrubbers have also been developed. These include venturi scrubbers in which the gas stream is accelerated through a venturi and a stream (or streams) of water is injected at the venturi throat to collide with the particulate. Venturi scrubbers are shown, for example in U.S. Pat. Nos 3,385,030; 3,490,204; 3,567,194; 3,582,050 and 3,812,656. The venturi scrubbers are effective for particulate down to a size of 1 to 2 microns. If basic reagents are added to the scrubbing water, substantial quantities of acid gases may be removed. However, the venturi scrubbers are energy intensive and are typically operated at a pressure drop of 30 to 45 inches of water or higher. Not surprisingly, the effectiveness of the venturi scrubbers increases as more energy is supplied.
During the 1970's a number of improvements were made in the wet scrubbing technology. Ejector driven or fan driven scrubbers employing centrifugal separators or separated flow separators were developed which included the first use of the mixing capability of the free jet nozzle. Such apparatus is shown, for example, in U.S. Pat. Nos. 3,852,408; 3,852,409 and 4,141,701. Due to the development of much smaller droplets which were more effectively mixed with the gas stream, both particulate and acid gases were collected simultaneously with very high efficiency. Though far more efficient than the venturi scrubbers, these devices still required about 20-40 inches of water pressure drop to collect the desired amount of pollutants. In common with other wet scrubbing systems, the collection efficiency increased as the amount of energy delivered to the system increased.
The art has also developed pollution control systems that represent a combination of earlier developed devices. See, for example, U.S. Pat. No. 3,894,851. Thus it has been common to use a spray chamber followed by a cyclone separator or a venturi scrubber; a venturi jet scrubber followed by a separator; or two venturi jet scrubbers followed by a separator. U.S. Pat. No. 3,852,408 discloses a system for removing particulate and gaseous sulfur dioxide (or other acid gases) comprising a spray chamber for conditioning the polluted gas stream and removing large particulate, a hot-water drive and a chemical injection unit for driving the gas and capturing the remaining particulate and sulfur dioxide in water droplets, means for enlarging the droplets, and a cyclone separator for separating the water droplets containing the particulate and sulfur dioxide reaction products from the stream of cleaned gas. A similar system is shown in U.S. Pat. No. 3,852,409 wherein the driving system utilizes a steam ejector and a water spray in place of the hot water drive A still further development is shown in U.S. Pat. No. 4,141,701 which discloses a drive system employing supersonic steam, air, or gas ejectors or subsonic free jet nozzles as the drive unit and an aerodynamic flow separating system to separate the pollutant-containing water drops from the cleaned gas. Although the systems disclosed in U.S. Pat. Nos. 3,852,408; 3,852,409 and 4,141,704 are capable of removing substantially all of the acidic gases contained in a stream of polluted gas, the acidic gases are entrapped by water and this necessitates water treatment facilities.
In order to avoid the water treatment problem, attempts have been made to employ dry or semi-dry processes to remove acidic gases, principally sulfur dioxide. One approach has been to prepare a slurry of ground lime and water and to spray the slurry into a chamber containing hot contaminated gases. This technique, known as spray drying, has the advantage that the resulting product is a dry product comprising gypsum which may be easily collected, along with other particulate, in a baghouse. However, collection of more than 70-80% of the acid gases has not been feasible. Moreover, the efficiency of the utilization of the basic reagent, e.g., hydrated lime or limestone, is relatively low. Sodium-based reagents are generally more reactive but also more expensive.
Dry injection systems involving the pneumatic introduction of a dry, powdery alkaline material into a flue gas stream have also been suggested. In these systems, the alkaline material is usually injected upstream from an electrostatic precipitator or baghouse. Sodium based sorbents such as sodium carbonate, sodium bicarbonate, nahcolite (a mineral containing about 80% sodium bicarbonate) or trona ore which contains both sodium carbonate and sodium bicarbonate have been used. Lime has heretofore been tested in a dry injection system but has not been demonstrated with much success. Kelly, Mary E. and Shareef, S. A. Third Survey of Dry SO.sub.2 Control Systems (1981), p. 6.